Recovery from hybrid breakdown in a marine invertebrate is faster, stronger and more repeatable under environmental stress

Understanding how environmental stress alters the consequences of hybridization is important, because the rate of hybridization and the likelihood of hybrid speciation both appear elevated in harsh, disturbed or marginal habitats. We assessed fitness, morphometrics and molecular genetic composition over 14 generations of hybridization between two highly divergent populations of the marine copepod Tigriopus californicus. Replicated, experimental hybrid populations in both control and high-salinity conditions showed a decline in fitness, followed by a recovery. Recovery was faster in the salinity stress treatment, returning to parental levels up to two generations earlier than in the control. This recovery was stable in the high-salinity treatment, whereas in the control treatment, fitness dropped back below parental levels at the final time point. Recovery in the high-salinity treatment was also stronger in terms of competitive fitness and heat-shock tolerance. Finally, consequences of hybridization were more repeatable under salinity stress, where among-replicate variance for survivorship and molecular genetic composition was lower than in the control treatment. In a system with low effective population sizes (estimates ranged from 17 to 63), where genetic drift might be expected to be the predominate force, strong selection under harsh environmental conditions apparently promoted faster, stronger and more repeatable recovery from depressed hybrid fitness.